A pressure sensitive adhesive (PSA) (also known as “self-adhesive” or “self stick adhesive”) is a non-reactive adhesive which forms a bond at room temperature with a variety of dissimilar surfaces when light pressure is applied. No solvent, heat or radiation is needed to activate the adhesive. It finds wide applications in pressure-sensitive tapes and/or foils, general purpose labels, note pads, automobile trim, packaging, medical, and a wide variety of other products.
Nowadays, most commercially available PSAs are derived from rubber, acrylic, modified acrylic, and silicone-based formulations, which are predominately made from petrochemical-based polymers. Given that petroleum and natural gas are depleting nonrenewable resources with naturally limited supply and increasing cost, it is desirable to provide PSA compositions that can be made from renewable natural materials. In addition, it is highly desirable that the manufacture and use of PSA do not generate environmental pollution.
Vegetable oils are one of the most abundant renewable materials. Approximately 20 billion pounds are produced annually in the United States. At present, less than 600 million pounds of soybean oil is used in industrial application. Therefore, there is plenty of soybean oil available for new industrial uses.
Vegetable oil is mainly a mixture of triglycerides with varying composition of long-chain saturated and unsaturated fatty acids depending on the plant, the crop, and the growing conditions. The double bonds in unsaturated fatty acids may be converted into more reactive oxirane moieties (the epoxy functional groups) by appropriate reactions. Epoxidized vegetable oils (EVOs) such as epoxidized soybean oil (ESO) are commercially available and are widely used in rubbers, plastics, resins, coatings, and various thermosetting composites.
The use of vegetable oils as starting materials for making PSAs has numerous advantages such as low cost, low toxicity, inherent biodegradability, and fairly high purity. Three general approaches for making PSAs from vegetable oils have been disclosed (see WO 2008/144703). In the first approach, free-radically polymerizable functional groups such as acrylate or methacrylate groups are first introduced onto fatty acid, fatty esters or vegetable oils and then polymerized via a free radical polymerization method such as ultraviolet radiation to form PSAs. The introduction of the functional groups is typically accomplished through the reaction between epoxidized fatty esters or epoxidized oils and acrylic acid/methacrylic acid. During the polymerization, various acrylic or methacrylic monomers may be used to co-polymerize with acrylated fatty esters/fatty acids/oils. In this approach, petrochemical-based acrylate is still used. This approach is considered as an extension of traditional free radical polymerization methods of making petrochemical-based PSAs. In the second approach, fatty ester or vegetable oils are first epoxidized. The epoxidized fatty esters or epoxidized vegetable oils are then polymerized to form PSAs through cationically catalyzed ring-opening polymerization of the epoxy rings. Some other epoxy compounds may be copolymerized with epoxidized fatty esters or epoxidized vegetable oils for improving the properties of PSAs. The third approach involves the direct polymerization of carbon-carbon double bonds on fatty acids, fatty esters or vegetable oils with other free-radically polymerizable compounds such as acrylate or methacrylate. Unlike those in drying oils such as Tung oil, most carbon-carbon double bonds in vegetable oils are not conjugated, thus having relatively low reactivity during the free radical polymerization. In this third approach, fatty acids, fatty esters or vegetable oils have to be modified to form conjugated double bonds before the free radical polymerization.